This invention relates to communicating with, decoding, localizing, and counting of unmodified transponders in the presence of wireless collisions.
Electronic toll collection transponders (e.g., E-ZPass) are a widely used wireless technology. According to some estimates, 70% to 89% of cars in the United States include such transponders.
The transponders are usually simple devices including battery-powered radio frequency identification (RFID) circuitry. The most common use of electronic toll collection transponders is to quickly and automatically collect tolls for vehicles utilizing toll roads. In general, a toll for a vehicle is collected when the vehicle traverses a toll collection area (e.g., a toll booth) to enter and/or exit a toll road. To collect the toll, a RFID reader at the toll collection area emits a query signal which causes the transponder carried by the vehicle to emit a response signal including, among other attributes, a unique identifier for the transponder. A toll is charged to a customer (e.g., the owner of the vehicle) who is associated with the unique identifier.
Since there is no media access control (MAC) protocol for electronic toll collection transponders, the RFID readers at the toll collection area use highly directional antennas in isolated areas to ensure that responses emitted form multiple transponders are not received by the readers.